1. Field of the Invention
The present invention relates to a fuel cell system using, for example, a solid polymer membrane for an electrolyte membrane, and more specifically relates to a technique which humidifies the solid polymer membrane.
2. Description of Related Art
A solid polymer type fuel cell comprises a stack (hereinafter referred to as a xe2x80x9cfuel cell stackxe2x80x9d or a xe2x80x9cfuel cellxe2x80x9d) constituted by laminating a plurality of cells formed by placing a solid polymer electrolyte membrane between an anode and a cathode. Such a solid polymer type fuel cell generates by supplying hydrogen to the anode as a fuel, and air to the cathode as an oxidant so that hydrogen ions generated in a catalytic reaction at the anode passage through the solid polymer electrolyte membrane and move to the cathode, to thereby cause an electrochemical reaction at the cathode.
In order to maintain high generating efficiency, it is necessary to maintain the solid polymer electrolyte membrane in a saturated water content condition to thereby ensure a function as an ion conductive electrolyte membrane.
Therefore, a fuel cell system proposed for example in U.S. Pat. No. 5,543,238, comprises; an ejector which mixes fuel side exhaust gas exhausted from a fuel cell with fresh fuel gas newly supplied to the fuel cell and circulating the mixed gas to the fuel cell, and a humidifying apparatus arranged between this ejector and a fuel gas supply apparatus which humidifies the fresh fuel gas supplied to the ejector.
With this fuel cell, the vapor concentration (vapor partial pressure) of the fresh fuel gas supplied to the ejector is increased by the humidifying apparatus, the fresh fuel gas and the fuel side exhaust gas are mixed in the ejector, and the humidified mixed fuel gas is supplied to the fuel cell.
As in the above described related art, in the case where the fuel side exhaust gas exhausted from the fuel cell is utilized for circulation, a predetermined upper limit is set for the specific consumption of the fuel gas in the fuel cell, depending on the structure for flowing fuel gas to the inside of the fuel cell, the flow rate of the exhaust gas necessary for exhausting water generated inside of the fuel cell, and the properties of a catalyst constituting the fuel cell and the solid polymer electrolyte membrane. The specific consumption of the fuel is equal to a reciprocal of the stoichiometry (circulated amount of the fuel gas).
If the specific consumption of the fuel gas supplied to the fuel cell is made higher than the upper limit, a pressure difference of the fuel gas between cells in the vicinity of the supply port of the fuel gas and cells in the vicinity of the exhaust port increases within the fuel cell, and hence the output power from each cell constituting the fuel cell becomes nonuniform. Moreover, since generation of heat occurs at the time when the hydrogen ion generated from the fuel gas goes through the solid polymer electrolyte membrane, if the pressure difference of the fuel gas increases, the distributed heat source also becomes nonuniform in each cell, and for example, it becomes difficult to predict the life of the fuel cell. Hence, there is a possibility that it is difficult to maintain the performance of the fuel cell constant.
Accordingly, in the ejector, it is necessary to ensure a predetermined stoichiometry for the fuel gas. This stoichiometry is defined as a ratio of a flow rate Q1 of the fresh fuel gas introduced to the ejector to a flow rate Qa (=flow rate Q1 of the introduced fresh fuel gas+flow rate Q2 of the fuel side exhaust gas) of the mixed fuel gas exhausted from the ejector (Qa/Q1). As the water content contained in the fuel side exhaust gas increases, the partial pressure of the fuel gas contained in the fuel side exhaust gas decreases by the partial pressure of water, and hence the stoichiometry of the fuel gas itself cannot be sufficiently ensured.
Moreover, in the case where a predetermined stoichiometry is ensured in the ejector, the capacity for circulating the fuel side exhaust gas can be increased by, for example, setting a small nozzle diameter for of the ejector ejecting the fresh fuel gas, to thereby reduce the flow rate Q1 of the fresh fuel gas. In this case, however, a pressure loss for before and after the ejector increases.
Since the water vapor content which can be contained in the fuel gas increases, with a decrease of pressure of the fuel gas, for example, even if the fuel gas is in a high pressure condition with the relative humidity being 100%, after the fuel gas passes through the ejector and becomes in a low pressure condition, for example, the relative humidity decreases to 80%.
That is to say, if a humidifying apparatus is provided on the upstream side of the ejector, even if the relative humidity of the fuel gas is 100% before being introduced to the ejector, after the fuel gas passes through the ejector and becomes a low pressure condition, the relative humidity decreases, and there is a case where the humidified amount required for the fuel cell stack may not be satisfied.
As the pressure of the fresh fuel gas before being introduced to the ejector is set high, taking into consideration a pressure loss in the ejector for ensuring a predetermined anode-cathode pressure difference required between the anode and the cathode of the fuel cell, the water vapor content which can be contained in the fresh fuel gas decreases, thereby making it difficult to ensure the humidified amount required for the fuel cell stack.
In view of the above situation, it is an object of the present invention to provide a fuel cell system which can ensure a predetermined stoichiometry and a predetermined humidified amount required for the fuel cell, at the time of circulating and using the exhaust gas exhausted from the fuel cell.
In order to achieve the above object, the fuel cell system according to the present invention comprises: a fuel cell which generates power by an electrochemical reaction with a fuel gas supplied thereto; an ejector which mixes fuel side exhaust gas exhausted from said fuel cell with a fresh fuel gas, to generate mixed fuel gas, and circulating this mixed fuel gas to said fuel cell; and a humidifying device which humidifies said mixed fuel gas with water content contained in said exhaust gas, by bringing exhaust gas exhausted from said fuel cell into contact with said mixed fuel gas via a water permeable membrane.
According to the above described fuel cell system, the exhaust gas exhausted from the fuel cell (for example, the fuel side exhaust gas or the oxidant side exhaust gas) is used as the humidifying gas which humidifies the mixed fuel gas after having passed the ejector. The water content contained in the exhaust gas passes through membrane holes in, for example, a hollow fiber membrane and is diffused as water vapor in the mixed fuel gas.
As described above, since water content is added to the mixed fuel gas having a relatively low pressure and humidified on the downstream side of the ejector, much more water content can be added, compared to the case where water content is added to the fresh fuel gas having a relatively high pressure on the upstream side of the ejector. As a result, a decrease in the relative humidity attributable to a pressure loss of the fuel gas before and after passing through the ejector can be prevented, and the humidified amount required for the fuel cell can be reliably ensured.
In the case where the fuel side exhaust gas is used as the exhaust gas, the fuel side exhaust gas whose water content is reduced after passing through the humidifying device is added to the fresh fuel gas in the ejector. As a result, the concentration of the fuel gas contained in the fuel side exhaust gas increases, thereby enabling improvement in the stoichiometry of the fuel gas.